This invention relates to an improved polymer composition and a film blowing process particularly suited for utilizing the polymer compositions. More particularly, this invention relates to the manufacture of films from such polymers as poly(vinylidene chloride), PVDC,
Polymers, particularly vinylidene chloride polymers, are widely used in packaging. In myriad instances they are used in forms that require formation of a film from the polymer. The polymers are typically formed into various shapes, including films, while molten or semi-solid, in a state somewhat less crystalline or solid than the most crystalline or solid form of the polymer at room temperature. It is important to avoid holes and other discontinuities during the forming processes. The forming processes to which this invention is most applicable include those wherein a polymer mass or film is elongated, stretched or otherwise manipulated, especially manipulated to form a thinner part.
Among such processes, film blowing processes, especially double bubble processes, are especially sensitive to formation of discontinuities because the blown bubbles necessary in the process fail, that is they break, become irregular in thickness, adopt irregular shapes, collapse or the like, all of which lead to eventual breaks, collapse or to discontinuation of the blowing process if the irregularity is noted before break or collapse. Efficient operation of a blowing process to produce a film of desirable uniformity in thickness requires that these discontinuities be avoided, that is, that the bubble remains stable during the blowing process. While the present invention addresses avoidance of discontinuities in other processes or other stages of processing, the effect will be referred to as “bubble stability” herein because sensitive tests on film-making bubbles can demonstrate the effect of the invention.
Bubble stability in other polymers such polymers as ethylene/vinyl acetate, has been improved by light crosslinking or coupling such as that taught in U.S. Pat. Nos. 4,515,745 and 6,506,848. However, these methods have eluded application to such polymers as vinylidene chloride polymers. It would be desirable to improve the bubble stability of polymers, especially vinylidene chloride polymers, by coupling or branching.
Branching to improve bubble stability is particularly applicable to formation of monolayer films because layers of polymers other than polyvinylidene chloride can compensate for tendencies to form discontinuities in a polyvinylidene chloride layer. Thus, there is a need for monolayer structures, particularly monolayer films and structures including packaging such as sheets, lidstock, and containers, for example, pouches, tubes and bags, especially those useful in high temperature applications such as bags suitable for hot-fill or in which foodstuffs can be cooked either at the time of packaging or by the consumer, for example, immersion in hot water or exposure to steam. Such thermal processing often is referred to as cook-in or retort, and films used in such processes are known as cook-in or retort films.